The catalytic properties of metals have been shown to vary with particle size. Metallic particles in the 1-5 nm range (essentially bulk metals) are traditionally prepared by reduction methods that produce a size distribution within each batch; meaning that the particle sizes within a population (or batch) can vary greatly. See, e.g., Sophie Carenco, Cedric Boissiere, Lionel Nicole, Clement Sanchez, Pascal Le Floch, and Nicolas Mezailles, “Controlled Design of Size-Tunable Monodisperse Nickel Nanoparticles”, Chem. Mater., 22 (2010) 1340-1349.
A relatively unexplored concept is the study of the properties of metal clusters/nanoparticles as the particles increase in size from a small number of atoms to larger particles that generally have the properties of bulk metallic solids. For example, in the case of gold (Au), bulkier particles of 50 nm and larger are conductive, non-catalytic, and non-toxic, but in the range of 5 to 10 nm the material becomes conductive, and at sizes less than 5 nm become insulating (in the case of Au32), fluorescent, toxic (Au55), and catalytic. Thus, when operating in the size regime of 1 nm and smaller, differences of a single atom may produce significant differences in the properties of each particle. This window or range is from metallic clusters to about 1 nm particles, or 2-100 atoms (for palladium, 50 atoms form a cube 0.9 nm on a side whereas 150 atoms form a 1.3 nm cube) where control of the number of atoms by direct synthesis is difficult yet the size is small enough where one can expect to see substantial chemical differences upon adding or subtracting even one atom. To date, there has been no general tool to prepare macroscopic amounts of small metallic clusters with an arbitrary, pre-selected number of atoms.
Smaller metallic clusters (tens of atoms) are amenable to direct synthesis and have been detected, however with the exceptions of certain “magic-number clusters” they are difficult to prepare in bulk and thus have not been isolated in macroscopic quantities, and accordingly have not been well characterized. See Zhikun Wu, Joseph Suhan, and Rongchao Jin, “One-pot synthesis of atomically monodisperse, thiol-functionalized Au25 nanoclusters,” J. Mater. Chem., 19 (2009) 622-626.
“Magic-number clusters” refer to certain very particular forms of atomically defined nanoparticles that can be prepared by virtue of their inherent chemistry, such Au25 and Au55 as well as C60 and C70, and can be distinguished from the “atomic metrons” described herein which permit arbitrary design of atomically-defined nanoparticles. With a magic-number cluster, one must accept the atomic composition imposed by nature.
Monodisperse “magic-number cluster” nanoparticles have been prepared and studied in the gas phase. Richard E. Smalley received the Nobel Prize in 1996 for C60 discovered by this procedure, however it is still a very complex procedure to make and requires substantial purification to isolate C60 and C70 from other soot components. For example, M. E. Geusic, M. D. Morse, and R. E. Smalley, “Hydrogen chemisorption on transition metal clusters”, J. Chem. Phys. 82 (1985) 5218-5228. Metal clusters of cobalt and niobium were prepared by bombarding metal surfaces with a laser and expanding the resultant clusters into a vacuum. The clusters were size-selected from the distribution formed and reacted through collisions in the gas phase with hydrogen. The authors showed selective reaction for different sizes, sometimes exhibiting great reactivity differences with only one additional atom. This approach is well known in the art, but you are essentially selecting the clusters one at a time and disposing of the vast number of clusters that do not meet the size criteria. This approach is not very material efficient. The clusters can be landed on a surface and isolated in detectable amounts but not macroscopic amounts by this technique without heroic effort and they do not necessarily retain their original atomic number.
A dendrimer is a repetitively branched molecule, so that the number of functional groups present in a dendrimer is limited to particular integers, increasing by powers of 2. One example of a dendrimer is poly(amidoamine). Dendrimers have been used to bind metal ions, but previously have not been used for the synthesis of homogenous populations of nanoparticles each composed of the same, specific number of atoms.
A group including Richard Crooks published on the use of dendrimers as templates for nanoparticle formation. See Marc R. Knecht, Michael G. Weir, V. Sue Myers, William D. Pyrz, Heechang Ye, Valeri Petkov, Douglas J. Buttrey, Anatoly I. Frenkel, and Richard M. Crooks, “Synthesis and Characterization of Pt Dendrimer-Encapsulated Nanoparticles: Effect of the Template on Nanoparticle Formation”, Chem. Mater., 2008, 20 (16), 5218-5228 and Valeri Petkov, Nick Bedford, Marc R. Knecht, Michael G. Weir, Richard M. Crooks, Wenjie Tang, Graeme Henkelman, and Anatoly Frenkel, “Periodicity and Atomic Ordering in Nanosized Particles of Crystals”, J. Phys. Chem. C 2008, 112, 8907-8911. They used dendrimers as templates and varies the number of metal ions that they contained by varying the ratio of metal ions to dendrimer. However, none of these dendrimers are decorated with metal binding sites of strong affinity (Kd>1010) so that the dendrimers cannot measure-out the appropriate numbers of metal atoms, and thus the resultant nanoparticles include a relatively broad distribution of sizes.
Researchers in Japan have described the preparation of a coordinating dendrimer with binding sites for platinum in a step-wise fashion, as seen in Kimihisa Yamamoto, Takane Imaoka, Wang-Jae Chun, Osamu Enoki, Hideaki Katoh, Masahiro Takenaga, and Atsunori Sonoi, “Size specific catalytic activity of platinum clusters enhances oxygen reduction reactions”, Nature Chemistry, 1 (2009) 397; Daigo Yamamoto, Satoshi Watanabe, and Minoru T. Miyahara, “Coordination and Reduction Processes in the Synthesis of Dendrimer-Encapsulated Pt Nanoparticles”, Langmuir 26 (2010) 2339-2345; and Yousuke Ochi, Kozue Sakurai, Keisuke Azuma, and Kimihisa Yamamoto, “Phenylazomethine Dendrimers with Soft Aliphatic Units as Metal-Storage Nanocapsules and Their Self-Assembled Structures”, European Journal Chemistry-A, 17 (2011) 800-809. However, the approach taken was to fill the binding sites by controlling the ratio of platinum to the dendrimer. The binding is different for different parts of the dendrimer so that different “layers” are filled depending on the ratio of platinum to dendrimer. No purification nor removal of excess metal ions were done, likely because the binding was too weak and ions would be undesirably extracted.
As described in P. G. Z. Benini, B. R. McGarvey, and D. W. Franco, “Functionalization of PAMAM dendrimers with [RuIII(EDTA)(H2O)]”, Nitric Oxide 19 (2008) 245-251, preformed Ru (EDTA) was bound to a commercially-available poly(amidoamine) dendrimers using carbodiimide chemistry. This group used chemistry that showed that an EDTA metal chelate could be attached to dendrimers. The complex was not purified. It was used for nitrous oxide reduction studies and was not reduced to a nanoparticle.
One group has attached an isothiocynate chelator to an amine terminated dendrimer and bound it to Gd+3, as disclosed in L. D. Margerum, B. K. Campion, M. Koo, N. Shargill, J. LAI, A. Marumoto, and P. C. Sontum, “Gadolinium(III) DO3A macrocycles and polyethylene glycol coupled to dendrimers: effect of molecular weight on physical and biological properties of macromolecular magnetic resonance imaging contrast agents”, J. Alloy. Compd., 249 (1997) 185-190. After binding the gadolinium, the material was dialyzed to remove free Gd. The resultant molecules were not purified nor were single entities as evidenced by FAB-MS. The chelated dendrimer carrier was used for in vivo studies as an NMR contrast agent. No nanoparticles were produced.
A need exists for a technique to select specific numbers of atoms per nanoparticle so that macroscopic batches of nanoparticles can be obtained.